Psychological stress is associated with increased risk of several health conditions including cardiovascular disease, autoimmune disorders, infectious disease, and mental illnesses (B. S. McEwen, Stress, adaptation, and disease. Allostasis and allostatic load, Ann. N. Y. Acad. Sci. 840 (1998) 33-44). The link between psychological stress and physical ailment can be seen in the biological responses associated with stress, namely the production of cortisol, a major glucocorticoid in humans.
Cortisol is synthesized and secreted by the zona fasciculata and the zona reticularis of the adrenal cortex. In its free form, cortisol plays an important role in the regulation of various factors such as, for example, blood pressure, glucose levels, and carbohydrate metabolism (R. Fraser, M. C. Ingram, N. H. Anderson, C. Morrison, E. Davies, J. M. C. Connell, Cortisol Effects on Body Mass, Blood Pressure, and Cholesterol in the General Population, Hypertension. 33 (1999) 1364-1368; D. S. Charney, Psychobiological Mechanism of Resilience and Vulnerability: Implications for Successful Adaptation to Extreme Stress, Am. J. Psychiatry. 161 (2004) 195-216). Cortisol levels can increase by ten-fold following surgery or other major trauma, as the steroid acts to prevent vascular collapse, reduce inflammation, and suppress immune response.
Additionally, cortisol, also referred to as hydrocortisone, is clinically used as a steroidal anti-inflammatory drug, and may be used in the treatment of, for example, acute inflammation, chronic inflammation, autoimmune diseases, allergic diseases, shock, gout, acute leukemia, and allograft rejection. However, chronically elevated cortisol is associated with the neuroendocrine causal pathway linking environmental or psychological distress to poor health outcomes (M. van Eck, H. Berkhof, N. Nicolson, J. Sulon, The effects of perceived stress, traits, mood states, and stressful daily events on salivary cortisol, Psychosom. Med. 58 447-58).
One of the major limitations of currently available cortisol immunoassay kits and immunosensors is their cross-reactivity and interference with the cortisol structural analogs, namely, progesterone and prednisolone (S. Tunn, G. Pappert, P. Willnow, M. Krieg, Multicentre evaluation of an enzyme-immunoassay for cortisol determination, Clin. Chem. Clin. Biochem. 28 (1990) 929-35; I. A. Ionita, D. M. Fast, F. Akhlaghi, Development of a sensitive and selective method for the quantitative analysis of cortisol, cortisone, prednisolone and prednisone in human plasma, J. Chromatogr. B. Analyt. Technol. Biomed. Life Sci. 877 (2009) 765-72). As recently reviewed by Krasowski et al., commercially available cortisol immunoassay kits still have cross-reactivity with the analogs, especially prednisolone (M. D. Krasowski, D. Drees, C. S. Morris, J. Maakestad, J. L. Blau, S. Ekins, Cross-reactivity of steroid hormone immunoassays: clinical significance and two-dimensional molecular similarity prediction, BMC Clin. Pathol. 14 (2014) 33). Additionally, conventional electrochemical biosensors suffer from low detection sensitivity that requires signal amplification for improved performance.
Sensors based on electrochemical processes can be used to detect a chemical or biological substance by using a transducing element to convert a detection event into a signal for processing and/or display. Biosensors employ biomolecules such as enzymes, antibodies, and nucleic acids as the sensing component. Alternatively, synthetic molecular biosensors can be configured to use specific chemical properties or molecular recognition mechanisms to identify target analytes. Biosensors can use the transducer element to transform a signal resulting from the detection of an analyte by the biologically sensitive component into a different signal that can be addressed by optical, electronic or other means.
Advantageously, electrochemical biosensors offer real-time sensing of clinically important biomolecules at low-cost and minimal power requirements ideal for decentralized point-of-care facilities and implantable or hand-held devices.
A biosensor typically comprises a bioreceptor and a signal transducer and is able to selectively sense the presence of a biological analyte of interest. Bioreceptors can be a variety of biological or chemical agents including, for example, nucleic acids, cells, antibodies, and enzymes. These bioreceptors can selectively react with, and bind to, a specific target analyte such as small chemical molecules, proteins, cells, DNA, and toxins. Conventional signal transducing methods employ various physical and chemical mechanisms, such as electrochemical, fluorescence, optics, and piezo-electricity.
The majority of currently-available biosensor techniques for the detection of target molecules rely on the use of natural antibodies and enzymes. Antibody-based biosensors possess a number of limitations such as inability for continuous monitoring, high cost of production, and need for special handling and storage, which altogether constitute roadblocks for commercialization of them as point-of-care diagnostic tools.
Therefore, there remains a need for developing sensitive biosensors for detecting and quantifying cortisol in a biological sample without requiring cumbersome handling and storage procedures.